Many-Body Effects and Photo-Induced Spatially Condensed Phases in the Exciton Systems in Layered Crystals

我们为本项目制备了高质量的BiI_3、GaSe和PbI_2层状晶体。在这些晶体中，我们研究了与多体效应有关的高密度激子的空间行为和一些新的光致凝聚相，如玻色-爱因斯坦凝聚（BEC）。在BiI_3中，称为层错激子（stacking fault excitons， SFE）的激子态出现在层错界面的准二维平面缺陷处。通过探测空间分辨泵浦探测光谱和空间分辨光致发光光谱，研究了n_2激光泵浦染料激光器激发SFE的空间行为。从这些光谱中证实了高密度激子在二维空间中集体相干运动的存在。我们用锁模Nd: YAG激光系统在皮秒时间范围内检测了这些光谱。事实证明，激子的一个组成部分与极化子模式的群速度相比，以一定的速度迅速膨胀。对于该系统以及GaSe激子，我们还应用了一种新的简并四波混频技术（DFWM），该技术使用空间隔离的两束激光在激子的共振能量下调谐。由于激子的传播，DFWM信号首先在两个激发光斑之间的中点被观测到。将观察到的SEE在高密度下的异常行为与基于非线性薛定谔方程理论的计算结果进行了比较，并讨论了其与玻色-爱因斯坦激子凝聚的关系。

We'v prepared good quality layered crystals BiI_3, GaSe, and PbI_2 for this project. In these crystals we have studied spatial behavior of the excitons at high density in connection with many-body effects and some new photo-induced condensed phases such as the Bose-Einstain condensation (BEC).In BiI_3, exciton states called the stacking fault excitons (SFE) appear at a quasi two-dimensional planar defect of a stacking fault interface. The spatial behavior of SFE excited with a N_2-laser-pumped dye-laser was investigated by detecting space-resolved pump-and-probe and space-resolved photoluminescence spectra. From these spectra, there is confirmed the existence of the high density excitons moving collectively and coherently in the two-dimensional space. We examined these spectra in the picosecond time region with a mode-locked Nd : YAG laser system. It turns out that a component of the excitons expands rapidly with a velocity compared to the group velocity of the polariton mode. For this system as well as GaSe excitons, we also applied a new technique of the degenerate four-wave-mixing (DFWM) with spatially isolated two laser beams tuned at the resonant energies of the excitons. The DFWM signals was observed firstly at the middle point between the two exciting laser spots due to the propagation of the excitons. The observed extraordinary behavior of the SEE at high density is compared with the calculation based on the theory composed of a nonlinear Schrodinger equation, and the connection with the Bose-Einstain condensation of excitons is discused.